Restoration Corner
Bruce Adams
Buy Your Classic for the Body, but Check the Engine
Restoration specialists often recommend that a prospective classic-car buyer inspect the body rather than the engine. And that advice is generally very true. A car with a solid body and little rust will be much more valuable than one requiring extensive rust repair. The engine is just another electrical/mechanical component to be evaluated during a restoration and can be rebuilt at any time during ownership.
But there is no need to rebuild an engine merely because the car is undergoing a complete, frame-up restoration. And when there is a limited budget, careful analysis can help determine how much to rebuild and the amount of work the engine will require, saving time and money.
Begin with Compression Testing
Analysis of the engine begins by observing its performance and characteristics after firing up the car. The 190SL pictured above is visible through a cloud of smoke, a clear indication that all is not working as the factory intended. By itself, though, this is not reason to rebuild the bottom and top half of the engine. The issue must be examined more carefully and a simple compression test is a reasonable first step.
An engine is basically an air pump requiring good compression to run effectively and start easily. A compression test checks the inner machinations of an engine without taking it apart. Two tools and methods can measure compression; manually, with a compression gauge, or electronically with an engine analyzer that measures cranking compression.
The test results will mean nothing unless they are referenced against manufacturer recommendations found in a service manual. The thing to look for in a compression check is comparable readings on all cylinders. If all the cylinders check out within approximately 10 pounds per square inch of each other and those numbers are close to factory specifications, that’s one indication the engine does not require rebuilding. If one or more cylinders measure a difference of 15 or more pounds per square inch, there are potential problems requiring disassembly to repair.
With electronic testing, a computer analyzer estimates compression in each of the engine’s cylinders by measuring slight variations in engine cranking speed. The results compare with actual gauge readings and can be completed in a matter of minutes without removing spark plugs. The analyzer prints the results of the test, comparing the actual numbers.
To check compression manually with a gauge, all spark plugs must be removed so the starter can spin the engine freely. The coil is disabled or the high-tension lead grounded, and the carburetor throttle must be held open. The engine is then cranked for a few seconds using a remote starter switch or an in-the-seat assistant while the manual compression gauge is held in a spark plug hole.
The maximum compression reading is recorded and the process is repeated for each remaining cylinder. The individual cylinder readings are then compared to manufacturer specifications. As a rule, most engines should have 140 to 160 pounds per square inch of cranking compression with no more than 10 percent difference between any cylinders.
If compression is low in one or more cylinders, the problem can be isolated to the valves or rings by squirting a little 30-weight motor oil into the cylinder through the spark plug hole and repeating the compression test. The oil temporarily seals the rings. If compression readings are higher in a second test, that means the rings and/or cylinder are worn. No change in the compression readings signals that the cylinder has a bad valve and/or valve seat.
Low compression in one cylinder usually indicates a bad exhaust valve. Low compression in two adjacent cylinders is typical of a bad head gasket, while low compression in all cylinders means the rings and cylinders are worn, an indication for an engine overhaul.
In preparing for an engine compression test, the battery should be in good shape, as it will have to spin the engine quite a few times. And a cold engine will give inaccurate compression readings, so warm the engine for more accurate results. Heat swells and seals all the metal parts inside the cylinders.
Further Analysis – Leakdown Testing
A leakdown or cylinder leakage test is similar to a compression test because it demonstrates how well an engine’s cylinders are sealing. Instead of measuring pressure, it measures pressure loss. In other words, a leakdown test is a compression test in reverse. Rather than measuring the engine’s ability to create pressure, compressed air is introduced into the cylinder through the spark plug hole. One gauge on the tester measures the air pressure entering the cylinder and the other measures the percentage of air escaping or leaking from the cylinder. The loss percentage indicates the cylinder’s condition and the engine’s condition overall.
A leakdown test requires removing all spark plugs and before compressed air is introduced into the engine, the cylinder being tested must be placed at top, dead center; the piston must be at the top of its travel; and intake and exhaust valves must be closed. Begin with cylinder number one and follow the engine’s firing order. A threaded coupling attached to a leakage gauge is screwed into a spark plug hole and compressed air – 80 to 90 pounds per square inch – is then fed into the cylinder. When the air is compressed into the cylinder, the leakdown gauge will measure any loss of air escaping past valves or piston rings. If the cylinder is not at top dead center, air escaping past an open valve will report a false reading.
No engine has perfect sealing with no percentage loss, but an engine in excellent condition should measure only 5 to 10 percent leakage. An engine with mileage but in acceptable condition may read up to 20 percent leakage. More than 30 percent leakage, however, indicates a potential problem that needs further inspection. The percent of leakage should also be consistent across the cylinders. Any great difference indicates a problem in that cylinder.
A leakdown test’s advantage compared with a compression test is that it’s faster and easier to determine where the pressure is going: If air is escaping from the tailpipe, that indicates a leaky exhaust valve; air coming from the throttle body or carburetor signals a leaky intake valve; and air leaking from the breather vent or positive crankcase ventilation (PCV) valve fitting indicates the rings and/or cylinders are worn. A leaking exhaust valve makes a hissing noise in the exhaust pipe while a leaking head gasket can cause bubbling in the cooling system.
The leakage test can be used in conjunction with a compression test to diagnose other kinds of problems. A cylinder that has poor compression but minimal leakage usually has a valve train problem, such as a worn cam lobe, broken valve spring, collapsed lifter or a bent push rod. If all cylinders have low compression but minimal leakage, the most likely cause is incorrect valve timing. Check whether the timing belt or chain is off a gear or two. If compression is good and leakage is minimal, but a cylinder is misfiring, there could be a fuel- delivery or ignition problem, such as a fouled spark plug or bad plug wire.
The Video Scope
A video borescope or inspection tool uses a miniature video camera at the end of a flexible tube that can be inserted into a spark plug hole. A display in the handle relays camera footage and enables mechanics to inspect the cylinders’ interior and piston tops without removing the engine head. This is another method used in our workshop when analyzing engines. We don’t use it often, but when required, it is an essential and useful tool. A scored cylinder or detonation problems can be located with minimal effort.
Don’t Jump to Conclusions
“When you have eliminated the impossible, whatever remains, however improbable, must be the truth,” Sherlock Holmes is quoted. Approach engine testing with a Holmes-like analysis – careful and deliberate – and don’t jump to conclusions. Consider all the facts the data reveals and consult other experts for their technical opinions. Then, and only then, proceed with an engine rebuild.
Bruce L. Adams is a Triangle Section member, and has owned and operated B.L. Adams, LLC, specializing in restoration of the 190SL, for 31 years. For more information, visit www.bruceadams190sl.com.
Captions:
Engine smoke indicates something in the engine needs to be fixed. Simple tests can tell you just what to look at.
Manual compression tester gauge on the right and leakdown tester on the left are simple to use, and useful to determine how bad engine problems might be.
The video borescope is an invaluable means for inspecting the inside of the engine block and other components, and is now available at a reasonable price.